How to add playing videos to the 3d viewport?
Create a plane, give it a material.
In the Shader Editor create this node chain: Texture Coordinate (Generated output socket) &gt; Image Texture &gt; Diffuse or Emission &gt; Material Output. You may also want to put a Mapping node between the Texture Coordinate and the Image Texture in order to move the movie in the plane.
In the Image Texture load your video, tweak some of its settings: enable Auto-Refresh and set the amount of frames your movie requires.

Or, as No-Can-Do says, enable the Import Images As Planes addon and press ShiftA &gt; Images &gt; Images As Planes
and open the Shader Editor to tweak the settings.
If your video is not a video file, but a series of numbered images. Press the gear icon when adding an image as plane, and select Animate Image Sequences,so that all the images are imported in a single plane and not as multiple planes, each one with a single frame.